The eukaryotic ribosome is composed of equimolar amounts of RNAs and proteins encoded by over 70 genes transcribed by three different polymerases. A major goal of this proposal is to elucidate mechanisms that coordinate the expression of some of these genes. Our recent observations suggest that specific translational regulation of ribosomal-protein mRNAs (rp-mRNAs) plays a major role in modulating ribosomal gene expression during early embryogenesis of Drosophila, when rRNA synthesis is undetectable. To study the interdependence of rRNA synthesis and rp-mRNA translation, bobbed mutants deficient in rRNA genes will be used to genetically reduce rRNA synthesis in the ovary (a site of intense ribosome synthesis) and investigate its effects on the expression of r-protein genes. A complementary series of experiments will address the issue of balanced r-protein gene expression by introducing cloned rp-genes into the embryo and examining at which level (e.g. rp-mRNA abundance, rp-mRNA translation, r-protein turnover) the embryo compensates for this excess dosage. To identify sequences on rp-mRNAs that confer their unique pattern of translation during early development, "hybrid genes" composed of varying portions of rp-mRNAs (whose translation is strongly regulated in early embryos) and actin mRNA (whose translation is not regulated) will be constructed. These "hybrid genes" will be introduced into early embryos and the association of the corresponding "hybrid mRNAs" with polysomes will be measured. These experiments should indicate which portion of the rp-mRNA confers upon it the ability to be translationally regulated. To study rp-genes by a genetic approach, a screen has been devised to isolate multiple mutants in individual r-protein genes. Each mutant will be analyzed at the molecular level by DNA sequencing. This combined genetic and molecular approach will constitute a first step in the identification of sequences in r-protein genes that are important for their expression and function. As a whole, these studies may serve as a model for other sets of genes that need to be coordinately regulated during development and cell differentiation.